The present invention is directed to a system for automatically steering a utility vehicle, wherein signal quality information from two position sensors is evaluated and used to steer the utility vehicle along an intended target path.
WO 94/24845 A and U.S. Pat. No. 6,128,574 disclose an automatic steering system for agricultural vehicles, wherein the system locates the vehicle on the basis of its immediate position and its intended target path. The position is determined from a location sensor receiving satellite location signals (GPS or DGPS). In this way the automatic steering system can continue to steer the vehicle even if the satellite location signals fail. U.S. Pat. No. 6,128,574 discloses that the utility vehicle is equipped with operating direction sensors and velocity sensors. Both references propose that the signals of the satellite system be supplemented by sensors attached to the utility vehicle, that can detect, for example, the crop edge of a standing crop or windrow. The crop edge can be detected, according to WO 94/24845 A, by an image operating system or, according to U.S. Pat. No. 6,128,574, by a reflex location system (such as, for example, a laser scanner) or by a harvested edge orientation system relying on mechanical contact with the crop.
In both cases cited above, the steering is performed exclusively on the basis of a map previously stored in memory, that defines the path to be followed. However, in some applications no map of the area to be processed may be available and the generation of a map would be uneconomical.
It is an object of the present invention to provide an improved automatic steering system for a utility vehicle that is highly accurate.
The automatic steering system of the present invention is provided with a first position sensor for generating a first position signal and a second position sensor for generating a second position signal. The first position sensor and the second position sensor are independent of one another. Both position signals are communicated to a controller having a memory. The controller is also supplied signal quality information about the quality of at least one of the position signals. The controller evaluates the position signals based on the signal quality information and weights the position signals accordingly to calculate the position of the vehicle and select the vehicle""s target path. The target path is selected from several target paths. The selected target path is the best path that corresponds to the position of the vehicle. Based on the position signal and the selected target path, the controller generates a steering signal that is communicated to a steering controller for steering the vehicle.
The controller can steer the vehicle along a target path defined by a fixed object (boundary of operation). The fixed object could be a crop edge that the vehicle is steered along during a harvesting operation. In this case the target path information corresponds to the intended position of the utility vehicle relative to the boundary of operation. A digital map generated in advance of the area to be processed is not required. If the signal quality information indicates the accuracy of the position signal is not adequate, the controller then derives the selected target path from previously recorded position information that was automatically recorded in the memory in the form of a map. Storing the position information is in the form of a learning operation. The steering then is controlled on the basis of the signal form the other position sensor. The target path information then corresponds to the map that was stored in memory to define the path to be followed. If it is found later on the basis of the signal quality information that the position signal detecting the boundary of operation is again adequate, the latter can again be used for steering the utility vehicle. Similarly, if the vehicle is steered from the satellite signals generated position signal along a path from a stored map, and the satellite signals cease, the other position sensor sensing the boundary of operation and a movement sensor can be used by the controller to generate a steering signal, the target path information now no longer corresponds to the map, but to the intended transverse distance to the boundary of operation.
In this way it becomes possible to equip an agricultural utility vehicle with several different position sensors and to automatically steer it on the basis of the most appropriate target path information conforming to the most appropriate position signals. The target path information is not necessarily derived from a pre-stored map, but from information that is relatively easy to obtain, for example, the desired position of a boundary of operation or information about a path that has already been covered.
In a preferred embodiment of the invention it is proposed that the controller gives weighted consideration to the position signals of the first and the second position sensor in the generation of the steering signal on the basis of the quality information that has been supplied to the controller. If the quality information points to a relatively high degree of accuracy of the first position signal, that first signal is considered exclusively or at least to a great proportion in the calculation of the steering signal. The second position signal is considered only in a small proportion or not at all. Analogously the first position signal is ignored or considered only in a small proportion, if the quality information points to a low accuracy of the first position information. With approximately median accuracy of the first and the second position signals, the first and the second position signal can be considered with equal weight. The target path information is selected accordingly.
Preferably the controller is provided with signal quality information for both position signals. The controller also uses this information, in order to establish which position signal is considered and to which degree it is considered.
The first position signal can be generated by satellite signals that can be generated by the GPS (global positioning system). However, the use of an inertial navigation system is also conceivable.
The second position signal can be generated by a local sensor on the agricultural utility vehicle. The local sensor can detect the movement of the utility vehicle (start, direction and velocity and possibly also the inclination of the terrain). Alternatively or in addition the local sensor is a sensor that can detect the position of the utility vehicle relative to a stationary object, particularly the boundary of an operation. Sensors of this type are sufficiently well known in the form of image processing systems, laser scanning sensors or mechanical scanning arrangements for the detection of rows of crop as are used in automatic steering systems for harvesting machines. Sensors for measurement of width of cut on cutter heads can also be used.
With laser scanning sensors there is occasionally the problem that the sensors can no longer recognize an edge. This problem occurs at that time, for example, when the edge is relatively low, several crop edges lie side-by-side, the harvesting machine rounds a curve or at the beginning of a harvest operation wherein the crop edge cannot be recognized. Therefore in the preferred embodiment, the controller stores previously position information indicating the path covered by the vehicle and the expected position of the boundary of operation. In that way if the headlands are harvested from the field at the beginning of a harvesting operation and if thereby the outline of the field is known to the controller by its record of the positions previously covered, then information is available as to which portion of the field has been harvested. This information can be utilized if the laser scanning sensor is not in the position to detect the boundary of operation automatically. This information can also be used upon the entry into the stand of the crop in order to establish the position of the boundary of operation so as to orient the sensor. The prediction of the controller relies on the assumption that the paths of the operation run steadily and parallel to the preceding path. Only edges corresponding to such paths are used by the laser scanning sensor as possible locations for the crop edge. Crop edges that extend at an angle or at implausible spacing distances from the previously detected path may be ignored. In order to recognize crop edges running at angles, all detected crop edges are stored in memory for a period of time so that the path of the vehicle along the crop edge can be recognized. If during operation around curves the boundary of the operation has reached beyond the measurement region of the laser scanning sensor, the angular scanning region of the laser scanning sensor can be oriented anew on the basis of the position detected. This is performed internally within the sensor by changing the position of lenses, transmitter and receiver and/or by rotating the entire sensor unit.
By estimating the position of the boundary of operation the scanning angle of a laser scanning sensor can be reduced by the controller to a region where the boundary of operation is expected. Thereby higher scanning rates and better control response can be attained at higher forward propulsion velocities. Nevertheless during the narrowing of the scanning sensor region the quality (accuracy) of the available position data must also be considered.